A class-forming operator that takes two
arguments: a variable and a formula containing at least one unbound
occurrence of the variable. The result of applying KappaFn to a
variable and a formula is the SetOrClass of things that satisfy the formula.
For example, we can denote the SetOrClass of prime numbers that are less
than 100 with the following expression: (KappaFn number
(and (instance number PrimeNumber) (lessThan number 100))). Note that
the use of this function is discouraged, since there is currently no
axiomatic support for it.